[meshcop] introduce new public APIs for SteeringData (#12148)

This commit introduces a set of public APIs to allow manipulation
of `otSteeringData`. The new APIs are provided when the configuration
`OPENTHREAD_CONFIG_MESHCOP_STEERING_DATA_API_ENABLE` is enabled.

The internal `SteeringData` is also improved to enhance robustness.
Methods such as `Init()`, `UpdateBloomFilter()` now return an `Error`
to signal failures on invalid arguments (e.g., invalid length)
instead of asserting.

src/core/thread/mle_ftd.cpp

@@ -2696,9 +2696,9 @@ void Mle::SetSteeringData(const Mac::ExtAddress *aExtAddress)
     {
         Mac::ExtAddress joinerId;
 
-        mSteeringData.Init();
+        IgnoreError(mSteeringData.Init(MeshCoP::SteeringData::kMaxLength));
         MeshCoP::ComputeJoinerId(*aExtAddress, joinerId);
-        mSteeringData.UpdateBloomFilter(joinerId);
+        IgnoreError(mSteeringData.UpdateBloomFilter(joinerId));
     }
 }
 #endif // OPENTHREAD_CONFIG_MLE_STEERING_DATA_SET_OOB_ENABLE